It is known in the art to provide security documents, such as for example ID, passports and other similar documents with different security features. A well known security feature is the watermark that is created during the creation of a paper substrate by specific process and that is embedded in the paper.
The main characteristics of a watermark on the level of security can be seen as the following: it can be a protection of the paper being fabricated to avoid a non-authorised reproduction (typically such as forgery), it is a safety feature being made by a technique that is not easily available, it is usually created by a specific tool, it can be controlled without specific means (for example visually), it has the possibility of a creation with multiple levels resulting in multiple grey tones.
These features have the consequence that the watermark is seen as a high value security feature that is used in nearly all the paper based documents that have to be protected against forgery, such as ID, passports and other similar documents.
In the past years, the request for documents with security elements or features has increased in particular in view of their long life, many paper based documents have been replaced by documents made of a synthetic material or of a combination of synthetic materials. Such documents are able to fulfil the need of a 10 years lifespan but comprise no watermarks since such an element is difficult to be created in a synthetic material.
Tests have been made to modulate the opacity of a document by cutting out a window in an intermediate or core layer of said document. However, such cuts have brought collapse regions in the surface and also uneven wear at the location of the cut. For these reasons these tests were unsuccessful.
It is also well known of the art to create a surface watermark by changing the surface relief or the surface texture. On synthetic documents, this is achieved by using a (hot) stamp or an impression plate (can also be achieved during batch lamination). Both have their drawback, mainly related to pricing, yield and control of the final quality of the surface.